In a world of melting ice caps, storm surges, and tropical cyclones, the most resilient cities aren’t the ones that fight the water back—but the ones that absorb it.

By Fred Pearce

The ramshackle river port of Khulna in southwest Bangladesh is one of the most flood-prone urban areas on Earth. The third-largest city in one of the world’s poorest and most populous nations is at constant risk of inundation. It lies 125 kilometers inland from the shores of the Indian Ocean. And yet a tenth of this city of 2 million people is flooded at least ten times a year on average.

When Cyclone Aila swept inland in May 2009, unleashing a tidal surge up the Pussur estuary, nearly 2,000 kilometers of embankments in Khulna and surrounding districts were damaged. Many have never been repaired. The city is almost literally disappearing beneath the water.

The conventional explanation is that rising sea levels due to climate change are leaving such places at ever-greater risk from storm surges. But that is only part of the story. Sea levels in the open ocean are rising by three millimeters a year, but high tides in Khulna and along the estuaries of southwest Bangladesh are rising six times faster.

The destruction by farmers of mangrove forests along the coast in the Sundarbans swamp is partly to blame for the ocean’s greater ferocity. The mangroves once provided a wide buffer to soak up the winds and cyclonic tidal surges. But a new theory is gaining ground—a theory with important implications for many coastal regions around the world as they face up to the risks of climate change.

It seems that across the lowlands of southern Bangladesh, embankments erected to protect people from the rising tides are often making the problem much worse. They are constricting and funneling tide flows, pushing them further inland, and amplifying the tidal range. Low tides may often be lower, but the high tides in places such as Khulna—and in other delta regions around the world, from the mouth of the Rhine in the Netherlands to the Mississippi in Louisiana—are higher.

The physics isn’t difficult to grasp. When a surge of water runs into a hard barrier in one place, it doesn’t simply dissipate; it often rushes with increased ferocity somewhere else. And, with the water more tightly straitjacketed, the funneling effects can be lethal. What’s more, hard barriers such as concrete seawalls, steel floodgates, and engineered embankments lack the one trait most needed in a world of melting ice caps and expanding oceans—flexibility.

And therein lie the beginnings of a sea change in the world of coastal engineering: hard is giving way to soft. Instead of fighting water with concrete and mud, new teams of geomorphologists, urban designers, and ecologists are experimenting with softer infrastructure—mangrove forests, salt marshes, and sand dunes—that moves with the water and absorbs it like a giant sponge. It turns out that the form of coastal defense most likely to prove resilient against those forces of nature that we have unleashed is—nature itself.

Traveling by boat on the great rivers that cross southern Bangladesh is an overwhelming experience. The forces of nature often dwarf the puny efforts of engineers to hold them back. Almost half the country is built on wide and constantly shifting delta flats where two of the greatest and most ferocious rivers on Earth, the Ganges and the Brahmaputra, reach the Indian Ocean.

Here, 30 million people are in harm’s way. Floods that would be front-page news in other countries barely register here. Thirty villages disappeared under water in July this year, after 20 kilometers of embankments collapsed. That made three paragraphs. “Several thousand marooned for the seventh time in the last four months,” ran another headline…